Bilayer magnetic device operating as a single layer device



1955 A. J. KOLK, JR., ETAL 3,213,431

BILAYER MAGNETIC DEVICE OPERATING AS A SINGLE LAYER DEVICE Filed D60. 21. 1960 0075/? ggy/c I MINER MAGNE TIC LA YER ll/O/V-CONDUCWVE BASE &

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United States Patent 3,213,431 BILAYER MAGNETIC DEVICE OPERATING AS A SINGLE LAYER DEVICE Anthony J. Kolk, Jr., Rolling Hills, Larry J. Douglas, Los

Angeles, and Donal A. Meier, Inglewood, Calif., assignors to The National Cash Register Company, Dayton,

Ohio, a corporation of Maryland Filed Dec. 21, 1960, Ser. No. 77,451 12 Claims. (Cl. 340-174) The present invention relates to magnetic devices as used in information storage for information-processing systems. More particularly, this invention relates to novel structures and processes in forming such bistable magnetic devices to improve the operating characteristics thereof.

In the field of bistable magnetic data-storage devices, much time and effort have been expended to produce an individual storage element which is capable of changing from either remanent state to the other in a very short interval of time while requiring a very small driving current. Also, because of space limitation, it is desirable that the individual data-storage elements be as small as is practicable. Of course, the major design limitation of these devices for data-storage or memory applications in computers and control devices is that the BH characteristic or hysteresis loop of the magnetic material used in its structure be quite rectangular.

In the copending application of Clinehens et al., Serial No. 802,494, filed March 27, 1959, and now abandoned in favor of continuation patent application Serial No. 143,018, filed September 29, 1961, and of Chow et al., Serial No. 30,362, filed May 19, 1960, there are disclosed bistable magnetic devices, and processes for making such devices, which consist of long slender mono-filaments on which are electrodeposited adherent iron-nickel film-like layers composed of from about 93% to 99% iron and from about 7% to 1% nickel. Such devices have characteristic curves with the desirable and requisite B /B ratio, i.e., high re-ctangularity, and are characterized by a switching time of approximately 50 millimicroseconds which is also very desirable. In general, the switching time (time for reversal of stable state) of the material used in forming those devices is shortened as the percentage of nickel in the composition is decreased. However, the coercivity of such a material is of the order of 15 oersteds which requires a drive pulse of about 400 milliamperes to effect a reversal of the state of the magnetic material. While usage of such a current is feasible in commercial devices, it is desirable to reduce the magnitude of the driving current whenever possible.

It is, therefore, the major object of this invention to provide a bistable magnetic device possessing the requisite degree of rectangularity of its hysteresis loop and an exteremely small, i.e., fast switching time, and which device is also characterized by small coercivity and thus a small required driving current.

Another object of the invention is the provision of a bistable magnetic device having an exteremely small switching time and a coercivity in the region below 10 oersteds.

Still another object of the invention is the provision of a bistable magnetic device having the characteristics described above, which device may be produced by a commercially feasible process.

As stated before, the desirable features in a magnetic storage device include a small switching or turnover time, low coercivity, and a high degree of rectangularity of the hysteresis loop. When iron-nickel. alloys are used, the entities which may be varied to obtain the desired characteristics include the nickel percentage and the thickness of the magnetic layer. While optimum turnover times have been obtained with an iron-nickel alloy containing 1% to 7% nickel, relatively lower coercivities can be obtained with iron-nickel alloys of the Permalloy type containing 30% to nickel. In an effort to achieve optimum values of both the turnover time and coercivity as well as maintain a high degree of rectangularity of the hysteresis loop, the present invention utilizes a combination of layers of two different alloys. A major feature of the present invention, then, resides in a structure which comprises a long slender monofilament base having an electrically conductive surface, a first adherent iron-nickel film-like layer electrodeposited on this surface, and a second adherent iron-nickel film-like layer electrodeposited over the first layer, where one of the layers is composed of from about 30% to about 90% nickel and 70% to 10% iron and the other layer is composed of from about 93% to 99% iron and from about 7% to 1% nickel. Furthermore, it has been found that optimum values of the limiting characteristics are best achieved when the layer containing 30% to 90% nickel has the thickness of from 500 angstroms to 4,000 angstroms and the layer containing 1% to 7% nickel has the thickness of approximately 4,000 angstroms, although the latter thickness may range from 3,000 to 5,000 angstroms.

Other objects, features and advantages of the invention will become apparent or made evident in the following description of the magnetic device, and explanation of the process by which it is formed, taken in conjunction with the appended claims and drawings in which:

FIG. 1 is an exaggerated pictorial representation of a bistable magnetic device incorporating features of the present invention with a set of conductor windings as would normally be applied;

FIG. 2 is a cross sectional view of the preferred embodiment of the invention wherein the monofilament base is formed of a copper-beryllium alloy wire;

FIG. 3 is a cross sectional view of an alternate embodiment of the invention wherein the base is formed of a glass monofilament coated with a very thin layer of silver;

FIG. 4 shows characteristic magnetization curves for a bistable magnetic device formed with a combination of magnetic layers having the properties disclosed by the present invention; and

FIG. 5 is a set of waveform diagrams representative of the input and output signals associated with the operation of the magnetic device of the present invention.

Referring first to FIGURES 1 and 2, the stiff resilient rod-like magnetic device is formed of a spring-like component or wire 10 of an alloy of copper and beryllium and two very thin adherent films or coatings 11 and 12 both of a magnetic material. The rod-like filament or wire 10 may be of the type which has been heat-treated to give it stiffness and resilience. The magnetic material of the inner thin film or layer 11, which is electrically deposited on the wire in accordance with the procedure hereinafter described, is of the Permalloy type and is composed of from about 30% to 90% nickel and from 70% to 10% iron, the preferred percentages being such as to possess a substantially rectangular magnetization hysteresis curve and also optimum low coercivity for which an alloy of this type is noted. The magnetic material in the outer thin layer 12, which is electrically deposited over inner layer 11 in accordance with the procedure to be described later, is composed essentially of a major proportion of iron and a minor proportion of nickel such as to possess a substantially rectangular magnetization hysteresis curve and also a turnover time of extremely short duration. The preferred proportions of iron to nickel of outer layer 12 are of the order of 97 parts of iron to 3 parts nickel, by weight; however, usable devices are producible with the proportion of iron in this outer layer in the range from 93% to 99% and the proportion of nickel in the range from 7% to 1%, all by weight (this material hereinafter will be referred to as the low nickel material). The relative percentages of iron and nickel may be varied for both inner layer 11 and outer layer 12 by variation of the proportion of iron and nickel salts in the respective electrolyte baths from which the magnetic materials are electroplated onto the respective surfaces. The copper-beryllium rod or wire base is of a fine gauge of the order of from to 50 mils in maximum cross-sectional diameter and the preferred form is a very springly straight wire of 10 mils diameter. The thicknesses of both inner layer 11 and outer layer 12 are governed by the conditions under which the layers are electrodeposited and the preferred thickness of inner layer 11 is in the range from 500 angstroms to 4,000 angstroms when this layer is of the Permalloy type with the preferred thickness of outer layer 12 being approximately 4,000 angstroms when the percentage of iron in outer layer 12 is within the range of 93% to 99% with 1% to 7% nickel by weight. Under certain conditions, it may be preferable to electrodeposit the latter magnetic material first as inner layer 11 and coating this inner layer with a magnetic material of the Permalloy type as outer layer 12. In this case, the thickness of inner layer 11 would be approximately 4,000 angstroms with the thickness of outer layer 12 being in the range from 500 angstroms to 4,000 angstroms. The various thicknesses as stated above are computed by indirect methods. To achieve minimum coercivity, the Permalloy thickness is approximately 4,000 angstroms.

While the rod-like base member may be as small as 5 mils or as large as 50 mils in diameter in special cases, the above given preferred dimensions have been found to provide devices having suflicient strength and rigidity to permit handling Without necessity for exercise of unusual care, and have excellent magnetic characteristics for use in data-storage arrays and matrices.

As shown in FIGURE 1, the bistable magnetic device, which may be of a suitably long length, is used in conjunction with a coil unit 13 or a series of such coil units which include a variety of plural-turn single-layer solenoid-coils arranged in generally concentric relationship and encircling a respective small length or portion of the outer magnetic film or layer 12. An exemplary coil unit, as shown, comprises two superposed coils such as 13a and 13b, each having a respective pair of coil leads such as 13a13a" and 13b'13b. The conductors are insulated and may be in the form of flat wires, stranded wires, parallel-laid flat wires or other suitable forms. When a plurality of coil units are employed, the units are spaced along the magnetic rod-like device sufficiently far apart to obviate undesirable mutual magnetic interactions such that each coil unit cooperates with its own respective portion of the magnetic films or layers. The general nature of the coil units and interactions between these units and the magnetic films are like or similar to those described and explained in the co-pending application of Donal A. Meier, Serial No. 728,739, filed April 15, 1958, and now abandoned in favor of continuationin-part patent application Serial No. 795,934, filed February 27, 1959, to which reference may be had for further details in respect to these matters.

Referring to FIGURE 3, an alternate embodiment of the invention employs a stiff non-magnetic electrically non-conducting base 10' on which is coated a very thin electrically conductive material 10a with an inner layer 11 of magnetic material electrodeposited thereon and an outer layer 12' of magnetic material being electrodeposited on the inner layer 11. Inner layer 11 and outer layer 12 are of the same compositions and thicknesses as described above for the preferred embodiment of the invention. The base structure 10 is preferably a single monofilament of a stiff resilient non-magnetic electrically non-conductive material, such as glass or quartz, and serves mainly as a strain free support for and upon which are applied the other elements of the magnetic device. The monofilament is of a small transverse dimension, the diameter of the exemplary structure being about 10 mils. The electrically conductive material is preferably a very thin film or layer of silver. A silver solution may be sprayed onto the monofilament concurrently With a reducer solution to form this thin film or layer. This silver substrate should possess an electrical resistance of the order of from 3.0 ohms per inch to 0.4 ohm per inch with any resistance within this range being satisfactory. However, to reduce electrical losses in the final device, the silver should be as thin as will still provide satisfactory conductivity for plating of a firmly adherent magnetic layer.

Referring now to the preferred embodiment of the invention, the thickness of layers 11 and 12 as well as the magnetic properties of these layers are in many ways dependent upon the conditions under which the layers are electroplated. While a variety of plating baths and procedures may be used, the preferred electrolytic plating bath and procedure for producing the present invention will now be disclosed and explained. Prior to being subjected to the electroplating action in the electrolytic bath, the copper-beryllium alloy Wire is subjected to a thorough cleaning in a suitable alkaline cleaning bath. Immediately following the alkaline bath, the copper-beryllium base rod or wire is thoroughly rinsed with distilled water. The wire is then immersed in a concentrated nitric acid solution for a period suflicient to remove a thin surface layer of the copper-beryllium alloy and the acid treatment is immediately followed by a thorough rinsing with distilled water. Immediately following the acid etch and rinse, to obviate chemical degradation of the surface of the wire, the latter is subjected to the electroplating action. The following procedure refers to the embodiment wherein inner layer 11 is of a material of the Permalloy type.

The preferred electrolyte for plating of the Permalloy layer consists essentially of a solution made up from 5 grams of FeSO .7I-I O, 18.6 grams of NiSO .6H O, 50 grams of NH Cl, and 1 gram of saccarine, per liter of solution. This solution is kept relatively stationary (i.e. stirring is not required) and the pH of the solution is in the range from 3.0 to 3.4. The anode is preferably of platinum with the copper-beryllium wire serving as the cathode and the current density is in the range from 1 to 5 amperes per square decimeter. The bath in general may be kept at room temperature.

In order to achieve a high degree of rectangularity of the hysteresis loop, it is desirable to place a magnetic field about the copper-beryllium wire during the plating process to orient the magnetic layer, which field is directed along the axis of the wire and has the strength of approximately 200 oersteds. The exposure time, depending upon the current density, should be of suflicient duration to allow the thickness of the layer to be within the range of 2,000 angstroms to 4,000 angstroms.

An alternative electrolyte bath for plating of the Permalloy layer may consist essentially of a solution made up from 5 grams of FeSO .7H O, 218 grams NiSO .6H O, 9.7 grams NaCl, 25 grams H per liter of solution with a small addition of saccarine (0.83 gram per liter) and sodium lauryl sulphate (0.43 gram per liter). The operating conditions under which the alternative bath is used are the same as those described for the preferred bath except that the pH of the solution is from 2.7 to 3.0 and vigorous stirring is employed during the plating. Other baths which are reported in the literature for electroplating of Permalloy would also be suitable; however, the baths which have been described herein are known to produce the desired thickness and percentages for the alloy specified.

After the plating of inner layer 11, the monofilament is removed from the plating solution and subjected to a thorough rinse with distilled water. Immediately following this rinse, the wire is subjected to a second electroplating action in Which the outer layer 12 is electrodeposited onto the surface of the inner layer 11. This second plating process and electrolyte are next explained for the low-nickel material.

The preferred electrolyte for plating of the low-nickel material is essentially a solution made up from 290 grams of FeCl .4H O, 12 grams of NiCl .6H O, and 238 grams of CaCl .2H O per liter of solution, with an addition of dilute I-ICl, if necessary, to bring the pH to a value of 1.00:0.05. Enough iron powder or iron wool to make certain that the solution is ferrous rather than ferric may be added to the electrolyte bath. One manner and procedure by which the second electroplating process may be accomplished is to draw the monofilament through a 3 inch long zone of contact with the electrolyte at a speed of about 5 inches per minute using a plating current of from 12 to 25 milliamperes at room temperature. While the exposure time and current density may be varied somewhat, the magnitude of each should be such as to produce the desired thickness of outer layer 12 of approximately 4,000 angstroms. A similar procedure may be employed for electroplating the Permalloy layer.

Immediately following the second electroplating, the plated wire is thoroughly rinsed with water, and immediately dried as by means of an acetone bath or spray. The dried device is then immediately given a protective coating to avoid oxidation or other degradation of the iron-nickel magnetic film or coating. This coating may be applied by dipping the dried device in a suitable moisture-proof self-curing resin, preferably a urethane resin. As soon as the resin or other protective coating has cured or dried, the magnetic device is ready for association with the coil units as a data-storage unit element.

When it is desired to employ the alternative embodiment of this invention, that is the monofilament glass base with a thin silver substrate, the plating processes are similar to those described above for the copper-beryllium monofilament wire with the exception of the preparation of the monofilament to receive inner layer 11 of the magnetic material of the Permalloy type. The process of applying the thin silver substrate will next be described.

Before applying the silver substrate, the glass monofilament is chemically cleaned (as by a wet sodium dichromate and sulfuric acid solution at 130 F. followed by a Water rinse) and the rod is then sensitized by dipping in stannous chloride solution. The monofilament is then moved through a jet or spray of a suitable silvering solution and a silver reducing solution which may be, for example, Peacock Concentrated Silver Solution and Silver Spray Reducing Solution, both marketed by Peacock Laboratories, Philadelphia, Pennsylvania. For this operation, the monofilament is preferably arranged for concurrent translation and axial rotation to produce a uniform silver layer on the surface of the monofilament. While the electrically conductive substrate of silver may be accumulated during a single slow pass through the spray region, a more uniform layer may be produced by drawing the rod through the spray quite rapidly, and immediately rinsing in a distilled water spray to remove any unreacted silver salts and repeating the procedure several times until a satisfactory depth or thickness of silver has been produced. As stated before, the silver substrate should be as thin as will still provide satisfactory conductivity for the subsequent plating processes. The silvered monofilament may then be removed to the electroplating bath in which inner layer 11 is electro-deposited onto the silvered surface. The subsequent electroplating processes will be similar to that described above for the copper-beryllium wire.

As described in regards to the preferred form of the invention utilizing the copper-beryllium wire, the electroplating processes may be inverted when the glass monofilament is used so that the Permalloy type material becomes outer layer 12 instead of inner layer 11.

With appropriate variations in the plating baths the percentage of iron in each alloy layer may be varied. The following examples of alloy combinations have been produced which are characterized by a low coercivity and fast switching time, respectively, of the order of 10 oersteds and 50 millimicroseconds (parts are by weight):

A Low nickel layer 98 Fe; 2 Ni Permalloy layer 20 Fe; Ni

B Low nickel layer Fe; 0 Ni Perrnalloy layer 20 Fe; 1 Co; 79 Ni C Low nickel layer 98 Fe; 2 Co Permalloy layer 20 Fe; 80 Ni In each of the above examples the thickness of each layer is approximately 4,000 angstroms. In Example A, the alloys were plated without an orienting magnetic field, while, in Examples B and C both oriented and unoriented combinations were produced.

In FIGURE 4, there is shown a typical set of hysteresis curves secured by an oscillographic recording of the results of driving a bistable magnetic device of the present invention. Curve U illustrates the threshold curve in which a large value of the magnetic field strength, H, causes no change in induction, B. Curve Z illustrates a large change in induction caused by less than twice the value of the field strength used in the case of curve U, the high degree of rectangularity of the saturation loop (Z) and the Large threshold value of field strength in which the induction does not change are features of considerable merit in the application of the device to memory and switching applications. As shown in FIGURE 4, the B /B ratio is in excess of 0.95. It will be noted that the coercivity, H for this magnetic layer is less than 10 oersteds and is approximately 8 oersteds.

Whereas commercial toroidal ferrite cores are characterized by turnover times of the order of /2 microsecond to 5 microseconds, a bistable magnetic device of the present invention is characterized by a turnover time of about 50 millimicroseconds as shown by the curve for the output voltage of such a device in FIGURE 5 which is a reproduction of actual oscillographic records. This output voltage is a result of applying a drive pulse of approximately 250 milliamperes as indicated in the upper portion of FIGURE 5. The turnover time of 50 millimicroseconds is of the order of at least 10 times faster than the turnover time of conventional ferrite toroids.

An important and unexpected result of invention, as shown by FIGURES 4 and 5, is that the hysteresis curve for this multilayer structure is characteristic of a homogeneous structure and the coercivity can be varied by varying the thickness of the usually low coercivity layer. However, a more important result of the combination of magnetic layers is the low turnover time of the combination which is not affected by the high turnover time of the Permalloy layer, this latter time being of the order of /2 microsecond to 5 microseconds. Suffice it to say that efforts have been made for some time to find a bistable magnetic device having the characteristics of the present invention and the materials which have been herein utilized in combination were found to fulfill the imposed requirements for the purposes for which this magnetic device is to be used. With the present disclosure in view, modifications of the invention will occur to those skilled in the art; and accordingly it is not desired to be limited to the exact details of the illustrated preferred embodiments.

What is claimed is:

1. A bistable magnetic device comprising a base having an electrically conductive surface, a first thin layer of substantially rectangular hysteresis loop magnetic material uniformly disposed over said surface and a second thin layer of a different substantially rectangular hysteresis loop magnetic material uniformly disposed over and in direct intimate contact with said first layer so that the multilayer exhibits a rectangular hysteresis characteristic substantially of the form obtained for a single layer film, one of the layers being of an iron alloy including at least 93% iron by weight and the other of said layers being of an iron alloy including no more than 70% iron by weight, the thicknesses of said layers being chosen so that the composite multilayer film has a substantially rectangular hysteresis loop with a coercivity intermediate the coercivities of said first and second magnetic materials.

2. A bistable magnetic device comprising a base having an electrically conductive surface, a first thin layer of magnetic material uniformly disposed over said surface and a second thin layer of a different magnetic material uniformly disposed over and in direct intimate contact with said first layer so that the multilayer exhibits a rectangular hysteresis characteristic substantially of the form obtained for a single layer film, one of the layers being composed of from about 30% to about 90% nickel and 70% to iron, and the other layer being composed from about 93% to 99% iron and from about 7% to 1% nickel, the thicknesses of said layers being chosen so that the composite multilayer film has a substantially rectangular hysteresis loop with a coercivity intermediate the coercivities of said first and second magnetic materials.

3. A bistable magnetic device comprising a long slender wire-like monofilament base having an electrically conductive surface, a first adherent iron-nickel film-like layer electrodeposited over said surface and composed from about 93% to 99% iron and from about 7% to 1% nickel, and a second adherent iron-nickel film-like layer electrodeposited over and in direct intimate contact with said first layer and composed from about to about 90% nickel and 70% to 10% iron, the thicknesses of said layers being chosen so that the composite multilayer film switches under the influence of an applied magnetic field substantially in the manner of a single layer film.

4. A bistable magnetic device comprising a long slender wire-like monofilament base having an electrically conductive surface, a first adherent iron-nickel film-like layer electrodeposited on said surface and composed of from about 30% to about 90% nickel and 70% to 10% iron and a second adherent iron-nickel film-like layer electrodeposited over said first layer and composed of from about 93% to 99% iron and from about 7% to 1% nickel, the thicknesses of said layers being chosen so that the composite multilayer film switches under the influence of an applied magnetic field substantially in the manner of a single layer film.

5. A bistable magnetic device according to claim 4 wherein the monofilament base is formed of a copperberyllium wire.

6. A bistable magnetic device according to claim 4 wherein the base is formed of a glass monofilament coated with a very thin layer of silver.

7. A bistable magnetic device according to claim 4 wherein the first layer has the thickness of from 500 angstroms to 4,000 angstroms and the second layer has the thickness of approximately 4,000 angstroms.

8. A multilayer bistable magnetic device including a substrate, at least a first thin layer of a first ferromagnetic material disposed on said substrate, and a second thin layer of a second ferromagnetic material disposed over said first thin layer so as to be in intimate direct contact therewith, the thicknesses of the layers being chosen so that the hysteresis curve of the multilayer film approximates that of a single layer film, one of said layers being composed of from 30% to 90% nickel and from to 10% iron, and the other of said layers being composed of from 93% to 99% iron and from 7% to 1% nickel.

9. The invention in accordance with claim 8, wherein said first and second ferromagnetic materials each has a substantially rectangular hysteresis characteristic, and wherein one of said ferromagnetic materials has a relatively low coercivity with respect to the other.

10. The invention in accordance with claim 9, wherein the relatively low coercivity ferromagnetic material has a switching time which is relatively slow with respect to the other, the ferromagnetic materials and the thicknesses of said layers being further chosen so that the composite multilayer has a substantially rectangular hysteresis loop with a switching time which is of the same order as the switching time of the relatively fast switching ferromagnetic material and a coercivity which is intermediate the relatively high and low coercivities of said ferromagnetic materials.

11. The invention in accordance with claim 8, wherein said device has at least one encircling solenoidal winding to permit an axial magnetic switching field to be applied to the multilayer film.

12. A bistable magnetic device comprising a long, slender wire-like base formed of a copper beryllium wire having a diameter of the order of 5 to 50 mils, a first iron-nickel film-like layer deposited on said copper beryllium wire and composed of from about 30% to about nickel and from about 70% to 10% iron, said first layer having a thickness in the range from approximately 500 to 4,000 angstroms, a second iron-nickel film-like layer deposited over said first layer in direct intimate contact therewith and composed of from about 93% to 99% iron and from about 7% to 1% nickel, said second layer having a thickness inthe range from about 3,000 to 5,000 angstroms, the resulting multilayer film exhibiting a hysteresis characteristic which is substantially of the form obtained for a single layer film, and at least one encircling solenoidal winding provided on said wirelike base to permit an axial magnetic switching field to be applied to said composite multilayer film.

References Cited by the Examiner UNITED STATES PATENTS 2,984,825 5/ 61 Fuller et al. 340-174 3,031,648 4/62 Haber et al 340-174 3,069,661 12/62 Gianola 340174 3,077,586 2/63 Ford 340174 3,083,353 3/63 Bobeck 340-474 FOREIGN PATENTS 845,605 8/ 60 Great Britain. 854,153 11/60 Great Britain.

IRVING L. SRAGOW, Primary Examiner.

JOHN F. BURNS, Examiner. 

1. A BISTABLE MAGNETIC DEVICE COMPRISING A BASE HAVING AN ELECTRICALLY CONDUCTIVE SURFACE, A FIRST THIN LAYER OF SUBSTANTIALLY RECTANGULAR HYSTERESIS LOOP MAGNETIC MATERIAL UNIFORMLY DISPOSED OVER SAID SURFACE AND A SECOND THIN LAYER OF A DIFFERENT SUBSTANTIALLY RECTANGULAR HYSTERESIS LOOP MAGNETIC MATERIAL UNIFORMLY DISPOSED OVER AND IN DIRECT INTIMATE CONTACT WITH SAID FIRST LAYER SO THAT THE MULTILAYER EXHIBITS A RECTANGULAR HYSTERESIS CHARACTERISTIC SUBSTANTIALLY OF THE FORM OBTAINED FOR A SINGLE LAYER FILM, ONE OF THE LAYERS BEING OF AN IRON ALLOY INCLUDING AT LEAST 90% IRON BY WEIGHT AND THE OTHER OF SAID LAYERS BEING OF AND IRON ALLOY INCLUDING NO MORE THAN 70% IRON BY WEIGHT, THE THICKNESSES OF SAID LAYERS BEING CHOSEN SO THAT THE COMPOSITE MULTILAYER FILM HAS A SUBSTANTIALLY RECTANGULAR HYSTERESIS LOOP WITH A COERCIVITY INTERMEDIATE THE COERCIVITIES OF SAID FIRST AND SECOND MAGNETIC MATERIALS. 